5 IAQ Implications of Switching from Cooling to Heating

Nov 16, 2021

As temperatures drop in the northern hemisphere, many areas are now switching from cooling mode to
heating mode. Before your attention gets pulled away by the busyness of the holiday season, it is worth
noting how this switch to heating can impact the indoor air quality (IAQ). In this article, we highlight five
indoor environmental concerns that arise each year when the weather turns cold and buildings switch to

  1. Carbon Monoxide
    When it turns cold outside, buildings fire up equipment after a long summer hiatus. This equipment may include boilers, furnaces, and fireplaces. Where there is fire, there is a risk of carbon monoxide if combustion gases are not properly exhausted via venting. Combustion appliances will have a flue or power vent to handle the carbon monoxide risks—but there can be unforeseen problems. For example, a flue may become detached allowing for combustion gases to enter the occupied space. Or the negative pressure caused by a furnace may cause a hot water heater to backdraft when both are running at the same time. When equipment gets fired up for the first time, it is important to measure carbon monoxide levels at the equipment and its venting.

  2. Dry Air
    In the winter, heated indoor air is dry. There is a widespread myth that one heating method (e.g. radiant heat) doesn’t make the air as dry as another method (e.g. forced hot air). But warming cold outdoor air will always result in dry outdoor air, no matter the heating method. In buildings without many humidity sources, the indoor relative humidity on a freezing day will regularly be below 20%.

    Dry indoor air causes well-known health impacts: dry eyes, dry skin, and a dry nose. But dry air also puts occupants at a higher risk for some viral infections such as influenza and COVID-19. Although the reasons aren’t entirely understood, many respiratory infections are more infective when the indoor air is dry. Perceived dry air is among the most abundant complaints in office environments.

  3. Mold
    To combat the effects of dry air, many buildings will operate humidifiers to aim for a relative humidity ranging from 40-60%. The problem is that poorly insulated walls may be colder than the dew point temperature of humidified air, which leads to condensation. For example, if the indoor air is heated to 75°F and humidified to 50%, condensation will form on any surface that is 55°F or colder. That might not be an issue when it’s a little cold outside—but this can become a big issue when it is below freezing. Condensation can form on the wall surface and be observed as dripping or streaking. Or condensation can form inside the walls unbeknownst to the occupants, until such a time that a musty odor can be perceived. Condensation leads to mold, which can have damaging effects on the structure and the occupants. We recommend having a professional evaluate your property to identify an ideal relative humidity for your particular building and climate zone.

  4. Radon
    Heated air is less dense and will rise in a building, creating vertical air movement called “stack effect”. Stack effect is strongest in tall buildings in the winter months when the indoor vs. outdoor temperature is greatest. The end result is positive pressure at the top of buildings and negative pressure at the bottom of buildings, which can result in radon being pulled in. Radon is a carcinogenic soil gas that is in high concentrations in some regions. Frozen ground may also lead to more radon being able to migrate into buildings at the lowest level. For these reasons, the winter is an ideal time to measure for radon. However, a recent study from the University of Calgary demonstrated that the winter increase in radon may not have as great an impact as originally thought.

  5. Inadequate Ventilation
    Many commercial buildings use economizers to bring in outdoor air. These controls allow buildings to bring in additional outdoor air on mild days. But these same controls revert the building back to minimum outdoor air settings on cold days. Many commercial buildings simply don’t bring in enough outdoor air ventilation with their minimum settings. Even homes may experience less ventilation in the winter. Many homeowners go from regularly keeping windows open in the fall to having the windows shut continuously for 5+ months.

    Inadequate ventilation is especially a concern in the winter because more activities move indoors. A school may do indoor recess and close windows to keep kids comfortable. More time inside a poorly ventilated class can increase the risk of respiratory disease transmission such as COVID-19 and influenza.

    It is always a good time to be thinking about indoor air quality. But when weather turns cold and the heat turns on, there are special risks to consider. Indoor Science would like to help you evaluate IAQ winter risks related to carbon monoxide, dry air, mold, radon, and inadequate ventilation. Our consultants and industrial hygienists can navigate you through these complex parameters to ensure a safe and healthy indoor environment this winter.